The present invention relates to an apparatus for switching control of an alternating current power source connected to alternating current loads such as AC-welders, magnetizers or elevators and, more particularly, to an apparatus for preventing the occurrence of possible transient phenomena in the circuit including an AC-power source and its loads owing to the interruption and, a short time later, recovery of the supply of electric power from the power source to the loads, for example, by the opening or breaking and a short time later re-making or reclosing of the circuit of the AC-power source, or by the occurrence of and, a short time later, recovery from a short circuit on the side of the power source.
AC loads such as mentioned above are often used in such a manner that the circuits of the power sources are repeatedly opened and closed at relatively short internals of time to accompany the occurrence of transient phenomena such as circulate a relatively high current through the circuit, depending upon circumstances. This may lead to the occurrence of accidents such as burning of circuit elements.
Assume that the load includes a transformer which has an iron core exhibiting a typical magnetic hysteresis loop. If the circuit including a commercial AC power source and its load connected thereto is broken or opened at a time when, for example, a first positive-going half of a cycle of the source current has elapsed, a residual magnetism will remain in the iron core. If the circuit is re-made or reclosed exactly at a time when a second, or immediately succeeding, negative-going half of the cycle has elapsed, the source current will flow for a third half cycle succeeding the second half cycle in the same sense as the current which has produced the residual magnetism in the core. Because the excitation impedance is very low because the residual magnetism acts to help the source current to flow in the circuit for the third half cycle, the source current instantaneously has a far higher value than its expected steady-state one. For a fourth half cycle succeeding the third half cycle and having the negative-going sense, the source current may now flow with a much lower magnitude than its steady-state one because the residual magnetism adversely acts on the source current. The residual magnitude will gradually decrease with the lapse of time. The alternately positive- and negative- going swings of the AC source current will gradually reach their steady-state values.
It is the same with the case where the circuit including the power source and the load is broken or opened at a time when the source current has depicted a first sinusoidal half of a cycle in the opposite sense, i.e., in the negative-going sense, and it is re-made or reclosed after the lapse of a second half cycle succeeding the first half one.
On the other hand, assume that a capacitive load is connected to the power source instead of the inductive load. If the circuit including the power source and the load is broken or opened and then re-made or reclosed before the charge stored in the load ends its self-discharge, a transient source current in excess of the expected steady-state value may flow in the circuit, provided that the source and the charged load have the same direction of electromotive force in the circuit.
In the case of inductive loads having relative large inductances such as iron-core transformers or induction motors, the time required for the residual magnetism to be lost often reaches 5 to 10 sec. or more. In the case of capacitive loads, the time required for the residual charge to be lost often reaches several to tens of seconds.
If the circuit including a power source and its load, inductive or capacitive, is broken at a time when a first half of a cycle of the source current, for example, in the positive-going sense ends and if it is then re-made or reclosed at a time when a whole cycle of the source current immediately succeeding the first half cycle has elapsed from the time when the circuit has been broken, the source current will begin to flow in the negative-going sense. In this case, the current will have the direction to cancel the residual magnetism thus far stored. Therefore, the excitation impedance is very high so as to produce no transient current.
Of the time elapsing from the breaking or opening of the circuit including a power source and its load, inductive or capacitive, to the re-making or reclosing thereof, is an odd number multiple of half of the period of the source current, a transient source current having a magnitude instantaneously larger than its expected steady-state value may flow in the circuit in a positive- or negative- going sense, depending upon the polarity of the residual magnetism. On the other hand, if the time from the breaking or opening of the circuit to the re-making or reclosing thereof is an integral multiple of the period of the source current, no transient source current would flow.
The above has been carried out on the example where the circuit including the power source and the load is broken or opened and then re-made or reclosed, respectively, at phase angles of the source current where the source current becomes zero, such as 180.degree. and 540.degree.. However, even when the circuit is broken or opened at any phase angle of the source current and then re-made or reclosed at another phase angle, similar results would be obtained so long as the difference between the phase angles of the voltage or current at the times of the breaking or opening and of the re-making or reclosing of the circuit is 360.degree. or its integral multiple.
It is the same with the case in which the supply of power, which has been interrupted owing to a short-circuit on the side of the power source, restarts in a short time.